Charge storage dissector tube



Dec. 27, 1938. P. T. .FARNSWORTH 2,141,835

' CHARGE STORAGE DISSECTOR TUBE Original Filed Jui 6/1935 LIGHT LIGHT g-3- INVENTORT 34 31 PH/LO 71 FARNSWORTH.

Patented n... 27,1938

CHARGE STORAGE DISSECTOR TUBE Philo '1. Farnsworth, San Francisco,Calif., assignor to Farnsworth Television Incorporated,

San Francisco, Oalii'., a corporation of California Continuation ofapplication Serial No. 30,118, July 6, 1935. This application September7,

1935, Serial No. 39,604

My invention relates to 'a charge storage dissector tube, and moreparticularly to a photoelectric tube adapted for electron beam analysis.

This application is a continuation of my contemporary application,Serial No. 30,118 filed July 6, 1935, for a Charge storage dissector.This latter application embodies a tube capable of utilizing the broadmethod disclosed and claimed by me in application, Serial No. 29,242,filed for an Electron image amplifier on July 1,1935, and

the present application describes and claims 'the tube structureutilizedin the system described and claimed in the first above mentionedapplication.

Among the objects of my invention are:

To provide an electron image amplifier tube adapted for use in electronbeam analysis; to

provide a sensitive television transmission tube;

to provide a tube which when energized will produce television signalsfrom objects illuminated with light of ordinary intensity; to provide aphotoelectric seal of increased output; to provide a televisiondissector tube utilizing a charge storage electrode; to provide acomposite charge storage electrode; to provide an amplifier ofphotoelectric currents; to provide an amplifier of photoelectricemission wherein amplification is accomplished of an electron imagebefore analysis; and to provide a simple and efiicient electron imageamplifying tube.

Other objects of my invention will be apparent or will be specificallypointed out in the description forming a part of this specification, butI do not limit myself to the embodiment of the invention hereindescribed, as various forms maybe adopted within the scope of the claim.

In my previous patents andapplications for United States Letters Patent,as follows: Patent Nos. 1,773,980, Aug. 26, 1930; 1,884,949, Feb. 16,1932, and 1,941,344, Dec. 26, 1933, and Serial No. 668,066, filed April.26, 1933,. and others, I have I described television transmittingapparatus and systems wherein an optical image of the object or picturefield is thrown upon a photosensitive cathode and the emitted electronsare accelerated and focused to form an electron image. By electron imageI mean a plane through which the electron stream passes, the electrondensity of which varies spatially across the stream in the same manneras the illumination density varies across the optical image. In otherwords, the

electron density values' represent spatially, the

illumination of the picture field.

The electron stream forming this image may be deflected by means wellknown in the art,

lated upon a radio wave, or transmitted by wire.

This method of television transmission offers the advantage of having nomoving parts and. of being suitable for the electrical transmission ofpictures having any desired fineness of detail.

The principal weakness of this method lies in the fact that only arelatively small portion of the electrons emitted from the totalphotoelectric area is used at any given instant and at the present timephotoelectric emission is relatively small in intrinsic value.Therefore, the highest possible sensitivity must be obtained from thephotoelectric surfaces and even then high gain amplifiers are necessaryin order that satisfactory picture currents may be obtained. With smalloutput currents, attempts to amplify the signals above a certain levelbring in background noise, shot effect due to the random velocities ofelectron emission and other ordinarily negligible factors which tend tomake the amplified picture currents unsatisfactory and distorted, andthe received picture lacking in the detail which it would have if suchinterference were not present.

In the present invention the fundamental principle of my previousinvention 'is retained and other desirable features added. An electronimage corresponding to the optical image is formed and is thereaftertreated as before. In the present device, however, the image has aconsiderably higher average value than in the previous devices becauseof the fact that space charges are formed, the electrons in the spacecharge being released by the action of the optical image. I am thereforeable to produce electron images with the present device which are farmore powerful than the electron images heretofore produced and underthese circumstances, when the image is.

scanned, picture currents of much greater amplitude are directlyobtained, thereby eliminating high gain amplifiers with theirobjectionable feaof the apparatus ofassociated applications.

In the specific embodiment of my method presented here, I prefer to forma charge image on an insulating medium and then pass a stream ofelectrons having uniform cross sectional density through the chargeimage to produce an electron image corresponding to the charge image andto the optical image responsible for the charge image. The charge imagereleases electrons from a space charge in the uniform stream to form theelectron image which is thus greatly more powerful than any image thatcould be 'created electrically. by the direct action of light onphotoelectric means.

Describing my invention broadly in terms of apparatus, I prefer toutilize a source of electrons preferably a filament mounted in anenclosing envelope and so positioned as to create an electron streamwhich is directed through a charge storage electrode on the way to acollecting anode. I prefer to form the collecting anode in the form ofan apertured electrode having a collecting electrode in back of theaperture so positioned that the electron image formed by the action ofthe charge image on the uniform electron stream may be scanned past theaperture to produce an analysis of the electron image, thereby creatinga train of television signals upon the collecting electrode.-

The present invention differs from my own previously described devicesof the same general type in that in the instant embodiment the chargestorage electrode serves both as a source of photoelectrons for chargingan insulating surface and also embodies the insulating surface itself;the photoelectric material being preferably on one side of the electrodeand the insulating material on which the charges are fixed beingpreferably on the opposite side of the electrode.

In my previous embodiments above referred to I have either projected anelectron image onto an insulating surface to project an image or I haveprovided the charge storage electrode with a photoelectric mosaic havingdiscrete islands which may be charged by the photoelectric emissiontherefrom in accordance with the light intensities falling upon them. Ihave developed the type of charge storage electrode with which thisparticular case is'concerned so that a charge image may be produced andfixed on an insulating medium forming a part of a photoelectric member.In other words, the charge storage electrode in this particularembodiment combines the insulating surface upon which charges are fixedand the photoelectric surface from which the energy comes to form thecharges. I also prefer to operate the device so that equilibrium isaccomplished independently of insulation leakage.

Various other modifications and applications of my invention will beapparent to those skilled in the art and for other broad aspects of myinvention I prefer to refer to a detailed description of severalpreferred embodiments of my invention as shown in the drawing, of which:

Figure 1 is a longitudinal sectional view of a television dissector tubeembodying my invention and provided with charge storage electrodesformed as indicated in Figures 2 and 3, which are enlarged crosssectional views of the individual wires of two preferred forms of chargestorage structures.

Figure 4 is a conventionalized diagram showing how the tube of Figure 1may be connected for operation.

Describing the apparatus in detail, envelope I is provided at one endwith a transparent window 2 through which an optical image may beprojected onto a charge storage electrode or grid I. The object isrepresented by an arrow 5 in Figure 4 and focused on a grid or chargestorage electrode 4 by a lens 6. Adjacent the transparent window 2 Iprefer to mount a source of flooding electrons I preferably in the formof a hot cathode either directly or indirectly heated. I prefer,however, to utilize a single filament with the leads 9 supporting thefilament placed one back of the other and passing through a stem III ina small side arm II. In this manner, the entire cathode structureobstructs very little light and therefore interferes inappreciably withthe optical image passing it. If desired, an accelerating anode may beutilized in combination with cathode 1. I also prefer to surround thefilament with an'annular film l2 surrounding the cathode, this filmbeing energized in any suitable manner to electrostatically control thepaths of the electrons. For the same reason, I prefer to surround thecharge storage electrode 4 with a grid band It preferably fixed to thewall of the tube and connected to the grid foundation wires.

The opposite end of the tube is provided with an axial extension IS inwhich is mounted an anode assembly comprising an anode tube IS providedwith an aperture i'l facing the filament 1 and the charge storageelectrode 4 and also provided with an interior collecting electrode I9positioned immediately back of the aperture. The anode tube i6 isprovided with an external lead 20 and the collecting electrode i9 isprovided with an external connection 2|. I also prefer to surround theanode end of the tube with an anode film 22 connected by means of link25 with the anode sleeve. The combined action of the films l2-l422creates an electrostatic lens as described and claimed in my priorapplication, Serial No. 30,118 filed July 6, 1935.

Among others, there are two structural embodiments which may be used inthe grid or charge storage electrode 4, both of which, how'- ever,accomplish the same function. Figure 2 shows one way in which the gridstructure may be made. Here a. foundation wire 25 is provided with auniform coating of insulating material '26 over the complete surfacethereof and the insulating material on the side of the grid structurefacing the window 2 is provided with a layer of photoelectric material21. In the other embodiment, shown in Figure 3, the foundation wireitself is preferably of silver 29 having a sensitized photoelectricsurface 30 thereon and the side of the charge storage electrode awayfrom the window 2 is provided with a coating of insulating material 26.Thus, either modification will have a photoelectric surface facing thewindow 2 and an insulating surface facing the anode apertur In formingthe charge storage electrode shown in Figure 2, I prefer to utilizenickel wire screen for the foundation 25 and form the insulating layer26 thereon by completely smoking the grid with the fumes of burningmagnesium to deposit a continuous layer of magnesium oxide. I thenevaporate onto one side of the electrode a thin layer of metallicsilver. This deposition may be accomplished by evaporation in vacuo asis well known in the art.

Oxidation of the silver film is then carried out preferably by using ahigh voltage glow discharge in oxygen and the oxidized surface is thensensitized with caesium, preferably with care being taken not to leavean excess of caesium. An excess of caesium can be prevented by bakingthe tube at sixty degrees on the pump after the photoelectric surface isformed, or by other means well known in the art; thus leaving asensitive photoelectric surface facing the window 2.

. In the second method of forming the charge storage electrode, thefoundation screen of the electrode is preferably of solid silver; inother words, the screen is woven of silver wire. The screen is thensmoked on one side only with magnesium oxide until it can be seen byexamination that the silver wires on the side of the screen facing anodeI 'l are fully covered with magnesium oxide with bare silver wire facingthe window 2.

The screen is then subjected to the glow discharge in oxygen, theexposed silver portion oxidized, and caesium deposited on this oxidizedsurface so that a continuously conductive photoelectric surface isformed facing the window 2. Thus, it will be seen that in bothembodiments a charge storage electrode is formed with a photo= electricsurface facing the clear window in a position to receive an opticalimage thereon, whereas the opposite face of the screen directed towardthe anode I1 is provided with an insulating surface.

In operation, the tube is hooked up in one preferred circuit as shown inFigure 4. Here the cathode is energized in any convenient manner and thecharge storage electrode 4 is maintained at a potential positive to thefilament I by means I of a grid source 3|. The anode tube l6 and thefilm 22 connected thereto are maintained at 'a positive potential togrid 4 by means of anode source 32 and the collecting electrode I9 ismaintained still more positive from the same source 32 and is connectedin series with an output resistor 34 across which output leads areconnected to tube IS. The television signal appears in the output leads,and is a measure of the difference between the electrons collected bythe tube and those passing through the aperture to be collected byelectrode IS.

The tube is provided with preferably magnetic deflecting means movingthe beam in two directions, coil 36 suppliedby its oscillator 3'!deflecting the image in one direction and coil 39, supplied by itsoscillator 40 deflecting the image in a direction preferably at rightangles thereto, thus causing scansion of the electron stream between thegrid 4 and the aperture H in two directions across the aperture.

In order that the electrons passing through the charge storage electrode4 be maintained in their electrical image relationship, I prefer toplace around the path of the electrons a focusing coil 4| supplied withcurrent by focusing source 42 under the control of a variable resistor44 to focus the electron image in the plane of the anode aperture. Thedevice is now ready for operation.

The object 5 is illuminated and the light reflected therefrom is focusedby means of the lens system 6 on to the charge storage electrode 4.

'Inasmuch as the surface of the charge storage liable to reformation inview of additional facts when obtained. Therefore, I do not wish to bebound by'all points of my present explanation although I believe it tobe substantially correct.

During operation, I prefer to operate the charge storage electrode at apositive potential. Let' us assume for a speciflc example a forty tofifty volt positive bias. Electrons from the flooding source I strikethe photoelectric layer with a primary velocity spectrum of from zero tosecondaries are therefore emitted; most of them; however, having a smallvelocity, say, under .5 volt,-b-ut a small percentage will have a fairlyhigh velocity even up to fifty volts, creating a secondary electronvelocity spectrum.

The secondary electrons are drawn through the meshes of the grid andsome of them strike the insulating surface thereof charging itnegatively to a potential which may be assumed to be from .5 to 1.0volt. If the average velocities complexity of the problem renders my'theory' of the secondaries which are emitted from the anode assemblythen assumes a minimum value.

When, however, the voltage on the grid is increased to a point where theanode current only approaches this minimum, the recovery after exposureof the photoelectric surface to light is' very slow and may take severalseconds. This behavior strongly indicates that the discharge of thecharges bound on the insulating surface of the grid is due to electronsfrom the source I rather than from any leakage through themsulation tothe foundation wire.

When the insulating surface is negative nearly all the floodingelectrons from the cathode 'l which strike the grid structure willstrike the conducting front surface. Those high velocity electrons whichdo not strike the front surface will have no tendency to strike theinsulating surface unless a portion of the insulating surface isdirectly in their path. The number, therefore striking the insulatorwill be quite small. Those which do strike, however, will cause emissionof secondaries from the insulating surface at a ratio greater than unityand will therefore discharge the insulating surface and create anequilibrium charge thereon independent of leakage.

The rate, however, at which the insulator is discharged will bedependent upon the amount of such primaries striking the insulatingsurface and the rate of discharge will be much slower. When theinsulating surface is made negative by photoelectrons from thephotoelectric surface, emitted under the influence of the optical image,the rate will also be proportional to the strength of the flooding beamfrom cathode I. As the rate will vary spatially in accordance with thephotoelectric emission, an electron image is formed in the streampassing through the grid to the anode assembly.

Inasmuch as it is very difficult to control the exact resistivity of theinsulating material which is placed on the back of the grid, it is muchmore satisfactory to operate the tube in a manner such as has been abovedescribed where the grid 7 is at a positive potential and where the highvelocity electrons from the flooding cathode I control the rate ofdischarge, rather than having to depend upon the more uncertain leakage.It is, therefore, a great beneflt in the operation of the device toutilize a flooding cathode and associated electrode structure which willgive a beam of flooding electrons having a maximum density of electronswithin a relatively low velocity category and a relatively smallernumber of electrons within higher velocity categories. Thus, the largemass of low velocity electrons can be utilized to form the space chargein front of the grid and the high velocity category can be utilizedpartly to create secondaries by impact with the photoelectric surface,which pass through the grid meshes to strike the insulator, thus givingit a negative charge, and partly to pass through the grid meshes toimpact the insulating material to eject secondaries therefrom therebyleaving a positive charge upon the insulator, the combination of the twocharges creating a uniform equilibrium charge which is a function of thevoltage between the grid and cathode.

Light falling on the photoelectric surface creates electrons which addto the equilibrium charge in greater or less degree in accordance withthe illumination in the optical image thus creating an electron imageback of the grid, the stream then being scanned to produce an amplifiedsignal train.

It is only then necessary to so adjust the grid voltage so that at themaximum negative potential occurring on the insulating material,sumcient high velocity electrons will penetrate the grid meshes, impactthe insulator and eject secondaries therefrom, to return the insulatorto equilibrium at that point within the time allowed for optical imageshift to the next picture fleld.

s,141,aso

It is, however, possible to operate the device with the grid foundationat a potential less than the point of unity secondary emission rationfor the insulator, by accelerating the flooding electrons by anauxiliary flooding anode until they enter deeply into the meshes of thegrid. Under these circumstances, it may be desirable to control theequilibrium charge by means of the leakage constant of the insulator orto utilize gas in the tube to aifect charge leakage. In any event, thecharge image is formed by the passage of photoelectrons from thephotoelectric surface to the insulating surface, the only diil'erence inoperation being the manner in which the insulating surface isdischarged.

I claim:

A thermionic tube comprising an envelope having a transparent window inone end thereof, a thermionic cathode of relatively small lightobstructing area positioned adjacent said window and adapted to emit acontinuous stream of electrons, an apertured grid of picture areapositioned to receive light passing through said window and passing bysaid cathode, said grid being coated on the side facing said window witha photosensitive material and being coated on the opposite side thereofwith an insulating material, means for drawing electronsphotoelectrically emitted from said photosensitive coating and electronsfrom said cathode through the apertures of said grid, means formaintaining the electrons drawn through said grid in a beam and inelectron image relationship, and means for collecting electrons insuccessive elemental cross sections of the beam to provide a signal.

Pmlb 'r. FARNBWOR'I'H.

